During forward motion of a rotorcraft, such as a helicopter, there is a difference between the relative air speed of an advancing rotor blade and a retreating rotor blade. As the lift generated by a rotor blade is dependent on the speed the rotor blade passes through the air, this imbalance in relative air speed tends to shift the centre of lift to one side of the rotorcraft. To maintain a balanced lift profile across the rotorcraft, the higher lift generated by the faster-moving advancing rotor blade is typically offset by decreasing its pitch (i.e. its angle of attack) relative to the retreating rotor blade. Nevertheless, the maximum forward speed of a rotorcraft may still be limited because the retreating blade may approach stall and/or the tip region of the advancing blade may approach a supersonic speed.
The pitch control on known rotor assemblies may also enable the rotorcraft to be manoeuvred by moving the centre of lift away from the axis of rotation. Known pitch control mechanisms can give rise to a number of disadvantages. For example: the mechanisms tend to be extremely complex and typically comprise various hinges and levers which have high maintenance costs and onerous safety requirements; excessive noise and vibration can occur because the pitch of the blade is altered, at relatively high frequency, during revolution of the rotor blades; and/or the rotor hub must be extremely strong as it is subjected to the intertial loads of the rotor blades and the loads associated with pitching the rotor blades, and it also transmits the lift force from the blades to the rotorcraft fuselage.